1. Field of the Invention
This invention relates to a sound reducing device which may be conveniently employed under extremely noisy conditions, such as in a vehicle or on construction sites, for reducing the external noise.
2. Prior Art
Up to now, a so-called ear-applying type noise reducing device has been extensively known for use in operations in extremely noisy places. This ear-applying type noise reducing device is attached to the user's head so as to be pressed onto side head portions, with a headphone cup covering the ears, to reduce the noise from the environment, and is used so that the noise from the environment is not intruded via a gap between the headphone cup and the side head portions.
More specifically, the inside of the cockpit of a helicopter or an aircraft represents an extremely noisy environment with the engine noises or the like which render it difficult for the pilot to recognize the contents of communications with the control tower. Thus the pilot is obliged to attach the headphone cup to his side head portions to reduce the noises from the environment.
Also, when the user intends to hear the playback sounds from a portable sound reproducing device with a headphone device in extremely noisy surroundings, the sound volume need be raised to elevate the playback sound level higher than the noise from the environment.
In addition, when one talks over a public telephone, the voice of the called party cannot be heard in noisy places, so that one has to raise the voice as much as possible in order to talk over the telephone.
However, since it is necessary with the above mentioned ear-applying type noise reducing device to apply the headphone cup strongly to one's side head portions in order to reduce the external sound otherwise intruded via a space between the headphone cup and the side head portions, one may feel constricted at one's head. Moreover, the headphone device itself is increased in size and weight and hence cannot be used for an extended period of time.
On the other hand, if the playback sound level is raised to higher than the external noise in order to render the playback sound easy to hear, the playback sound is leaked from the interstices between the ear and the headphone device to inconvenience near-by persons or to cause disturbances to the auditory sense because of the excessively raised sound level.
Additionally, a telephone is needed wherein one may talk without being bothered by external noises.
Meanwhile, as a sound reducing device for reducing the external noises, there is known an active type headphone device such as is shown and described in the U.S. Pat. Nos. 4,455,675, 4,494,074 and 4,644,581.
With this active type headphone device, a negative feedback loop is used whereby the electrical signals converted from the external noises by a microphone unit are fed back in a reverse phase for reducing the noise in the vicinity of the headphone unit.